How Waterfalls Form: Erosion, Rock Layers, and Retreat

Many textbook waterfalls develop where resistant rock overlies less resistant rock. River water and the sediment it carries erode the softer layer more quickly. Hydraulic action can force water and air into cracks, while abrasion happens when sand, pebbles, and larger fragments scrape and strike the channel.

As the softer rock is removed, the harder rock may be left jutting out above the drop. Water plunging over the edge gains speed and hits the riverbed below with considerable force. Swirling stones and sediment deepen a plunge pool at the base, concentrating erosion in one place.

This uneven erosion is the central idea. The river is not simply cutting straight downward at the same speed. Differences in rock resistance guide where the channel steepens and where undercutting develops.

Undercutting removes support from the harder rock above. Eventually part of the overhang can collapse under its own weight. The fallen blocks may then be broken up and used as additional abrasive material in the plunge pool, helping erosion continue.

After a collapse, the steep face of the waterfall has shifted a little farther upstream. Repeated undercutting and collapse can make the waterfall retreat over long periods. The old position is not left untouched. The river continues flowing through the abandoned section and may carve a narrow, steep-sided gorge.

This is why understanding waterfalls also explains a second landform. A waterfall and its gorge are often parts of the same changing river system. The waterfall marks the active step, while the gorge records some of the path along which that step has retreated.

Not every waterfall develops in exactly the same way. Useful factors to check include:

• •Rock type and structure, including joints, faults, and the thickness of resistant layers.
• •River discharge, because a larger flow can increase the energy available for erosion.
• •Sediment load, which can strengthen abrasion when particles strike the bed and banks.
• •Height of the drop, which affects the speed and force of falling water.
• •Local geology, because tectonic movement, glacial valleys, or sudden changes in rock can also create waterfall steps.

Waterfalls can also mark sudden changes created by past glaciation or tectonic movement, so the hard-rock-over-soft-rock model is not the only route. In a hanging valley, for example, a smaller tributary glacier may have eroded its valley less deeply than a larger main glacier, leaving a steep step after the ice disappeared. Use the local geology and landscape evidence rather than forcing every example into one diagram.

Field sketches are useful here because they force you to connect process with evidence. Label the resistant layer, softer rock, plunge pool, undercut section, and gorge, then add arrows showing the direction of retreat. A diagram becomes much easier to remember when every label explains a process rather than just naming a shape.

Niagara Falls is the classic textbook case, and its geology fits the model closely. A hard cap of dolomite lies over weaker shales and sandstones. The river undercuts the softer layers beneath, the dolomite overhang is left unsupported, and blocks break away. Since the falls began forming as the ice sheets retreated at the end of the last glaciation, this process has cut a gorge roughly eleven kilometres long between the present falls and the escarpment where the waterfall started.

In the past the falls retreated at a rate often estimated at around a metre a year. Today the rate is far lower, partly because a large share of the river is diverted through tunnels to hydroelectric power stations, especially at night and outside the tourist season, and partly because engineers have stabilised the rock. In 1969 the American Falls was even temporarily dried out behind a dam so the rock face could be studied. Human management now strongly influences how fast this natural landform moves.

Victoria Falls, on the Zambezi between Zambia and Zimbabwe, shows how cracks in rock can shape a waterfall as much as layering does. The river flows across a thick plateau of basalt, an old lava rock, which is crossed by joints that were filled with weaker material. The Zambezi wears that material out and pours into a long, narrow chasm, so the falls form a curtain well over a kilometre wide. Downstream, the river runs through a series of zigzag gorges. Each one marks an earlier position of the falls, cut back along a different line of weakness in the basalt.

By some measures the largest waterfall on Earth is not on land at all. Beneath the sea between Greenland and Iceland, very cold, dense water from the Nordic Seas spills over a submarine ridge in the Denmark Strait and sinks down the slope towards the deep North Atlantic, descending about three and a half kilometres. Estimates of its flow run to millions of cubic metres per second, far more than any river. This underwater cataract is driven by density rather than erosion. Cold, salty water is heavier than the warmer water around it, so it slides downhill along the seabed as part of the ocean's global circulation.

In Washington state in the United States lies Dry Falls, a cliff about 120 metres high and more than five kilometres wide, several times the width of Niagara. No river flows over it today. In the 1920s the geologist J Harlen Bretz argued that this landscape, part of a region called the Channeled Scablands, had been carved by enormous floods in a geologically brief time. Most geologists rejected the idea for decades, partly because it seemed to revive old catastrophe theories that the science of slow, gradual change had worked hard to replace.

The source of the water was later identified as glacial Lake Missoula, held back by a dam of ice near the end of the last ice age. When the dam failed, the lake drained in a sudden rush, and this happened repeatedly as the ice advanced again and the lake refilled. Evidence included giant ripple marks, several metres high, left by the draining water. At Dry Falls, the floodwater plunged over layers of basalt and cut back upstream in the same way as an ordinary waterfall, only far faster. Bretz lived to see his ideas accepted and received one of geology's highest awards in 1979, in his nineties.

How waterfalls form is mainly a story of river energy meeting uneven geology. Softer rock erodes faster, a plunge pool deepens, resistant rock becomes undercut, and collapses can move the waterfall upstream. Follow that repeated sequence and you can also explain the gorge left behind. Start with the rock layers, then trace what the moving water does to them.